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Driveline Problem

The car: 1990 Coupe Quattro, 130k mi. 
This is a generation II Quattro.
Here are the symptoms, not necessarily related:

The electronics for the rear differential lock have acted
flakey on a very few rare occasions. I can't tell whether
or not it is actually engaged, and it's response to the
manual button is erratic, accompanied by buzzing noise from
the general direction of the relay panel. The auto-disengage
never fails to operate at the appropriate speed, though. This
makes me wonder if there is more going on in the rear diff 
that I'm not aware of. Sort of a quasi-locked state which is
causing wear or damage. I don't really think this is an
issue, but I thought I'd throw it out there. 

Grinding noises emanating from the rear when starting off 
in a tight turn. It does not make the noise if the car is 
already in motion. The tires are NOT scrubbing when this
happens, however. That's an entirely different feeling.

Massive vibration on the highway between 65-75 mph.
Aggravated under load (uphill or accelerating), or
with extra weight in the car. Maybe coming from the rear 
drivers side. There are two periods of vibration. The first
is a rapid ~45/min shaking, which fades in and out with a
period of a few seconds. Odd. It almost feels like an 
unbalanced wheel.

A just barely detectable, very occasional, skipping or 
bucking at highway speeds. Just barely, bucking is much
too strong a word. Feels like bad gas, but it's definitely 
driveline. There is also a just barely detectable 
not-very-smoothness about getting the car in motion from
a full stop. It's starting to scare me.  

Prefer any BTDT over wild speculation.
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